This section provides background information related to the present disclosure which is not necessarily prior art.
Cells, which are hermetically protected, are located in large battery housings. As a rule, these cells are cooled by means of contact cooling. The temperature within the battery housing is thereby lower than the temperature outside of the battery housing. The contact cooling either comprises non-pressurized systems, which include water-based media, such as water-glycol mixtures or proper air conditioning systems, which operate with fluorocarbon or carbon dioxide.
Typical volumes of batteries, which are used in hybrid and electric vehicles, are more than 50 l (l=liter). Batteries of electric vehicles even encompass volumes, which are greater than 100 l.
Against this background, a battery housing typically contains an inevitable dead volume of approx. 5% of the entire volume of the battery. However, depending on the embodiment of the cells, this dead volume can also be considerably larger. In particular, geometrically disadvantageous round cells allow for only a relatively small use of space.
Slight excess pressures as well as low pressures can be created in a hermetically, in particular isochorically sealed battery housing due to pressure fluctuations, which are caused by temperature fluctuations. They are critical in particular for the sealing of the cells, which are installed in the battery, and impact the service life of the entire battery system.
Larger pressure fluctuations can furthermore have negative impacts on further battery components, in particular the electric power and control contacting of the cells as well as the seals of the housing lead-through of cables.
In the currently practiced embodiment [Lamm et al.: Lithium-Ionen-Batterie. Erster Serieneinsatz im S400 Hybrid (Lithium-Ion Battery. First series launch in S400 hybrid); ATZ 111 (2009); 490 ff], an available dead volume of approx. 2 l between the used round cells is filled with sealing compound. These sealing compounds, however, considerably increase the mass of the entire battery system, because 1 l of sealing compound weighs approx. 1.4 kg. In addition, this embodiment is expensive and its handling during the production is difficult and time-consuming, because the sealing compounds consisting of two components can cross-link and/or must be cross-linked. This embodiment furthermore prevents the partial exchange or replacement, respectively, of individual cells or modules during maintenance or repair.
Even though the mentioned embodiment can still be converted in the case of relatively small batteries, such as “mild hybrid batteries”, but leads to masses and costs, which are no longer operable in the case of large batteries, as they are used for pure electric vehicles.
In the event that the dead volume is not filled with solid or liquid substances, respectively, and the battery is in particular not sealed hermetically, temperature fluctuations can cause a material volume replacement with the surroundings. In response to the cool-down of the battery, it can suck in air. Moisture as well as dust can hereby be drawn into the interior of the battery. The moisture can condense in the interior of the battery. This is particularly critical because the water accumulation can lead to short-circuits and corrosion.
These replaced air volumes are low. In the case of a temperature difference of 50 K and a volume of 5 l, the replaced air volume is 50/300*5 l, thus approximately 1 l. The air flows occurring thereby are very low; that is to say, they are approximately 1 l/h.
A further demand on battery systems lies in that combustible gases, which are released in case of an emergency, either remain securely in the battery housing or are securely and specifically discharged or set, respectively.
Current embodiments use a permeable pressure compensation element in the battery housing. A pressure compensation element allows for a material replacement between the interior of the battery and the surroundings. Microporous membranes, for example made of “Gore-Tex” are currently used here above all, because they also effectively prevent the permeation of particles and liquid water in addition to a material replacement. In the case of such an embodiment, however, water can enter into the interior of the battery housing in a gaseous state and can condense there. The condensed water can then no longer reach towards the outside through the microporous membrane and thus accumulates in the interior of the housing.
This also leads to the danger of corrosion at the electric contacts as well as to short-circuits of the power electronics. Both lead to a breakdown of the battery.
The mentioned problem is even worse when, in addition to the condensed water, gaseous saliferous substances reach into the interior of the battery. For example, hydrogen chloride (HCl) reacts with the condensed water to form hydrochloric acid, which has an extremely corrosive effect. This can lead to pitting corrosion here. Furthermore, the electric conductivity of the water is increased drastically, which in turn increases the risk of electric short-circuits.
An improved embodiment uses the attachment of filters or filter and dehumidifying elements, respectively. With these measures, water can be caught even in the gaseous state and/or can be set. The water can thus not reach the interior of the battery.
In the case of these embodiments, it is disadvantageous that they do not permanently close the interior of the battery housing. The water-setting capacity is exhausted after a certain time. The dehumidifying element is thus a wear component, which must be replaced, depending on humidity and period of use. It must furthermore be absolutely ensured that the dehumidifying element is not contaminated with water, which is present at the outside. This water can appear as wade water in the case of vehicle batteries, in the case of vehicle batteries in high-pressure cleaners or in the case of washer systems.